1. Field of the Invention
The present invention generally relates to communications systems having duplicated communications systems.
2. Description of the Prior Art
FIG. 1 is a block diagram of a communications system having duplicated communications processing systems. The system shown in FIG. 1 has a duplicated configuration. Duplicated supervisory devices respectively supervise a system of a supervised object, such as a power feed system, and respectively send pieces of supervisory data to duplicated communications lines 2-1 and 2-2 via transmitter/receiver devices TX/RX. Then, the pieces of supervisory data are transferred to transmitter/receiver devices TX/RX connected to a host computer HC via communications processing systems 1-1, 1-2 and 1-3. The pieces of supervisory data transferred between the adjacent communications processing systems are indicated by reference number 3.
Each of the communications processing systems 1-1-1-3 comprises duplicated communications devices 11 and 12. Each of the communications devices 11 and 12 has a relay function of relaying data received via the corresponding communications line. Further, each of the communications devices 11 and 12 has additional functions. For example, each of the communications devices branches received data to another device or system (not shown), or extracts desired pieces of data from the received data for a specific use. For example, a display device (not shown) of each of the communications devices 11 and 12 displays necessary information on the basis of the extracted pieces of data.
The host computer HC supervises the status of the supervisory object on the basis of the supervisory information received from the transmitter/receiver devices TX/RX. Normally, pieces of identical supervisory data are separately transferred via the duplicated configuration. Even if one of the two systems malfunctions, necessary data can be transferred via the other system.
The host computer HC sends pieces of identical control data to the system via the transmitter/receiver devices TX/RX, the communications processing systems 1-3, 1-2 and 1-1, the transmitter/receiver devices TX/RX and the supervisory devices SV.
If a failure has occurred in the system to be supervised, an increased amount of supervisory data will be sent to the communications processing systems 1-1-1-3 from the supervisory devices SV via the transmitter/receivers TX/RX. In some cases, a large amount of supervisory data higher than a capability of each of the communications devices 11 and 12 may be generated. In these cases, each of the communications devices 11 and 12 fails to execute a process or processes having low priority, and is substantially down.
In order to reduce the loads on the communications processing systems to prevent the system from being down, some solutions have been proposed. For example, receipt of data is regulated when a buffer provided in each of the communications devices 11 and 12 becomes full of data. Alternatively, the communications devices 11 and 12 stop executing some processes, and share the processing of received data. However, even by the above solutions, it is impossible to completely prevent the generation of overloads of the communications devices 11 and 12.
With the above in mind, use of a watchdog timer has been proposed to rapidly recover the communications devices 11 and 12 from the down states. A watchdog timer is provided in each of the communications devices 11 and 12, and clears all data when the watchdog timer is not reset after a predetermined time. Then, default data is set in each of the communications devices 11 and 12, which are restarted by an initial program loading process.
However, in practice, both the communications devices 11 and 12 become down at almost the same time due to overloads arising from an increase in the amount of data. In this case, data cannot be transferred at all. The above fact greatly reduces reliability of the communications system.